The removal of single-phrase judgments “has to be a Government decision”, the chief inspector of Ofsted has said.

In a speech to school leaders, Sir Martyn Oliver said “so much” of the Government’s school improvement system in England rests on Ofsted grades.

Speaking about Ofsted ratings, which he called “the elephant in the room”, Sir Martyn said: “Whether grades stay, go or are reformed in the future, that decision will not stop us from building a better system now.”

At the annual conference of the NAHT school leaders’ union, Sir Martyn announced that Ofsted will scrap subject “deep dives” during ungraded school inspections from September, to “reduce the burden” on school leaders.

The education watchdog has come under greater scrutiny after the death of headteacher Ruth Perry.

Mrs Perry took her own life after an Ofsted report downgraded her Caversham Primary School in Reading from its highest rating, “outstanding”, to its lowest rating, “inadequate”, over safeguarding concerns.

Last week, the Department for Education (DfE) said it had no plans to remove single-phrase Ofsted judgments, despite calls for them to be scrapped.

In a speech to the NAHT’s annual conference in Newport in Wales on Saturday, Sir Martyn said: “I know some of you would like us to make changes to our gradings as part of a future system.”

He said: “We can’t do that unilaterally. So much of the Government’s school improvement system rests on our grades, that any changes would need to align with a bigger, wider remodelling of the whole accountability system.

“That has to be a government decision.”

In its response to an Education Select Committee inquiry on Ofsted, the DfE listed benefits of one-word judgments, which included providing a “succinct” summary for parents and helping to identify the support needed for schools.

Delegates at the NAHT annual conference have called on the union’s executive to “explore all campaign, legal and industrial routes” to secure inspection reforms to “safeguard leaders’ lives”.

An emergency motion on Ofsted, which was passed unanimously on Saturday, described the inspection system as “inhumane and unreliable”.

Sir Martyn called on headteachers to talk about the positives in schools as he said a focus on negatives risks “putting off” people from joining teaching.

In his speech to hundreds of school leaders on Saturday, the Ofsted boss said: “I do worry that if a narrative of negativity becomes all-encompassing that we may lose some of the best potential teachers in the next generation.

“Of course, we need to identify the problems, and be vocal about finding the solutions. Of course we do.

“But if that’s all we do, if we only talk about the negatives, the stresses, the things that make us want to give up, well then we risk putting off a generation of brilliant and inspiring teachers.”

On the watchdog’s plans to reform ungraded subject inspections, which check on standards in schools that are “good” or “outstanding”, Sir Martyn said: “I can announce today that we are acting to reduce the burden of inspection following feedback from smaller primary schools.

“We’ve heard from many of you that the inspection methodology is particularly challenging for these schools.”

He added: “It isn’t right and it isn’t helpful to try to cram all the detail of a full, graded inspection into an ungraded one. Instead, we want ungraded inspections to feel more like monitoring visits.”

In March, Sir Martyn launched the watchdog’s Big Listen public consultation, which closes at the end of this month, to seek views about the inspectorate.

On Friday, Paul Whiteman, general secretary of the NAHT, said the Government’s failure to address the damage being done by the current inspection system was “reckless”.

On the changes announced on Saturday, Mr Whiteman said: “The removal of deep dives from section eight inspections is a welcome and positive move.

“NAHT has long argued that the deep dive approach is not well suited to primary schools, and especially small primary schools, and we are pleased Ofsted has listened.

“We hope this is the start of an ongoing process of reform when it comes to how schools are inspected.”

A DfE spokesman said: “We have no plans to remove one-word judgments.

“They give parents the confidence in choosing the right school for their child and provide a clear basis for taking action to improve underperforming schools.

“The secretary of state has been clear that we will continue to consider ways to improve the current system, including looking at international approaches, and we are looking forward to hearing the views of teachers, parents and children through the Big Listen.”

Professor Julia Waters, the sister of Mrs Perry, said: “Sir Martyn Oliver has once again said that reforming school inspections will happen, but he still seems to be timid about the kind of reform that is necessary.

“It is not his words, but his actions and the actions of the Government which are most important now.”

She added: “Recent proclamations about school standards from the Education Secretary show that the current government seems to be sticking to single-word judgments as a way of prioritising spurious election gimmicks over serious care for teacher welfare and education.

“The Government seems to think that terrorising teachers, patronising parents and ignoring parliament is a vote-winner. We shall see if they are right soon enough.”